BYPASSING OF THE AQUEOUS PHASE BY METHANE GAS PRODUCED DURING CRUDE OIL BIODEGRADATION
A two-dimensional reactive transport model of oil degradation, production of CH4 and CO2, and outgassing of CH4 and CO2 from the aqueous phase was constrained by CO2 efflux data at the land surface and dissolved concentrations of CO2, CH4, and N2. The results predict that 86% of observed surface efflux consists of outgassed carbon that could not have cycled through the aqueous phase sampled in the monitoring wells. The separation of the effect of directly outgassed CH4 from the chemistry of the well samples suggests that there are isolated water pockets in contact with the degrading oil. These water pockets may be isolated by gas bubbles present in the oil body, which lower the relative water permeability, limiting the transport of water away from the oil. Outgassing from the aqueous phase is mostly in the form of CH4, which is consistent with CH4’s much lower solubility compared to CO2. Preferential outgassing of CH4 maintains low CH4 to CO2 aqueous concentration ratios, even when the plume is strongly methanogenic.